Conversion of sulfur dioxide to sulfur trioxide in an air mixture containing 7 to 10 vol % of SO.sub.2 to a degree of conversion of approximately 97 to 98% can easily be achieved by means of a number of commercially available vanadium catalysts promoted by postassium, sodium or mixtures of these alkali metals. Environmental protection laws in a number of industrialized countries require, however, that the off-gases leaving a sulfuric acid plant contain considerably less unconverted SO.sub.2 than corresponding to 97 to 98% conversion of a feed gas with 7 to 10 vol % SO.sub.2.
Hence, the prior art describes a number of procedures by which 99.5 to 99.7% conversion is obtained. Absorption of SO.sub.3 before the final pass of a converter chain is one well known design solution by which the thermodynamic equilibrium conditions can be made sufficiently favorable for attainment of 99.7% conversion of the original feed gas even though the lowest practicable operation temperature for the catalyst is usually said to be approximately 420.degree. C. Operation at elevated pressure with recirculation of off-gases from the last bed to the fresh-air drying towers can also, at least in principle remove all remaining SO.sub.2 by absorption in the sulfuric acid used as drying agent in these units.
The SO.sub.2 concentration in the off-gases can also be lowered by physical processes e.g. by adsorption on a dry sorbent or chemically by reduction of SO.sub.2 to elemental sulfur or by oxidation to SO.sub.3 using a solution of hydrogen peroxide in sulfuric acid.
The various double absorption plant designs and the process by which off-gases are recirculated are complicated and energy consuming compared to a straight through process. The use of a chemical such as H.sub.2 O.sub.2 to remove SO.sub.2 from the off-gases results in higher production costs of the sulfuric acid produced.
The ideal process would be one by which sufficiently high conversion (e.g. 99.7%) is obtained by a straight through conventional multipass conversion in a single absorption process using, for example, 4 to 5 catalyst passes.
In order to reduce the SO.sub.2 content of the off-gases to approximately 500 ppm, i.e. to attain a 99.5% conversion of a feed gas with 10 vol % SO.sub.2, in a process operating at substantially atmospheric pressure and without removal of SO.sub.3 by inter-pass absorption, the temperature of the gas leaving the final pass must not exceed 380.degree. to 385.degree. C. and is preferably 370.degree. C. or lower. This in turn requires that the inner temperature of the gas entering the final pass must not be higher than 370.degree. to 380.degree. C. and is preferably 365.degree. C. or lower.
The commonly used potassium promoted vanadium catalysts with K/V atomic ratios of 2.5:1 to 3.5:1, however, do not permit operation at these low temperatures. The reason for the dramatic decrease of activity of K (or Na) promoted catalysts below 415.degree.-420.degree. C. is discussed by Villadsen and Livbjerg (Catal. Rev.-Sci. Eng. 17(2) 203 (1978)). They reason that a certain inactive V.sup.4 species may precipitate from the catalytic melt at low temperature, thus rupturing the catalytic cycle V.sup.5 .fwdarw.V.sup.4 .fwdarw.V.sup.5 by which oxygen is transferred to SO.sub.2 to form SO.sub.3. Example 1 of this application shows the typical behaviour of a K promoted catalyst below 420.degree. C., and the results of this example may directly be compared to the results of the following examples 2, 3, and 4 where a catalyst identical to that of example 1 except that Cs is used instead of K, is shown to have high activity at a reaction temperature 70.degree.-80.degree. C. lower than the K promoted catalyst.
The ability of Cs to retain the activity of a V based SO.sub.2 oxidation catalyst at low temperature has been claimed in several patents starting with U.S. Pat. No. 1,941,426 by Beardsley et al (dated Dec. 26, 1933). This ability is postulated in a number of publications quoted in the above mentioned review by Villadsen and Livbjerg, further in a handbook on "The Manufacture of Sulphuric Acid", Reinhold 1959 pp. 171-176 and even in the recent U.S. Pat. Nos. 3,789,019 (issued Jan. 29, 1974) and 3,987,153 (issued Oct. 19, 1976) by A. B. Stiles.
There is considerable ambiguity concerning the role of Cs in the catalytic material. Stiles (U.S. Pat. No. 3,987,153) claims a catalytic activity of CsVO.sub.3, a compound that is unlikely to exist in the extremely acidic environment of the catalytic melt. Cs cannot participate in the catalytic process since it is unable to change oxidation stage. Its role is to stabilize a heavily sulfatized vanadium-oxygen complex ion, thereby preventing a gradual formation of inactive V.sup.4 species, the existence of which has been proved by ESR (electron spin resonance), IR (infrared absorption), and XRD (X ray diffraction) work.
Even though Examples 2-4 of this application unambiguously prove that a Cs promoted V catalyst is in principle different from a K promoted V catalyst in its low temperature behaviour it is, however, of considerable practical importance to improve the low temperature activity, since the activity of a solely Cs or Cs/K promoted vanadium catalyst--by the general decrease of rate constants with temperature--is significantly lower at 350.degree. C. than at the usual operation temperature 420.degree. C. of the last bed conventional-type catalyst.
It is generally agreed that the rate determining step of the SO.sub.2 oxidation process is the reoxidation of a V.sup.4 -containing ionic complex. It therefore occurred to me that improving the overall rate of absorption of oxygen in the melt would accelerate this catalytic step and thus the overall rate of conversion of SO.sub.2. Thus I concluded that it might be possible to improve the activity of the Cs or Cs/K promoted vanadium catalyst for SO.sub.2 oxidation by addition of any third component (second promoter) which improves the absorption of oxygen via oxygen ions to the complex V.sup.(4) -O-S ions that govern the reoxidation step of the catalytic cycle.
The catalyst of the present invention combines the beneficial properties of these two promoter effects to form a catalytic material with a low temperature activity many times higher than those described in the prior art.